Spread-spectrum communication systems are well known in the art and widely deployed. A class of receivers well-suited for spread-spectrum systems—such as IS-95, IS-2000 (cdma2000), and WCDMA—is a linear interference-whitening (LIW) receiver. LIW receivers suppress interference in addition to collecting signal energy. One form of LIW receiver is a transversal chip equalizer; another is a G-Rake receiver. The Rake receiver derives its name from its rake-like appearance, wherein multiple, parallel receiver fingers are used to receive multiple signal images in a received multipath signal. By coherently combining the finger outputs in a weighted Rake combiner, the conventional Rake receiver can use multipath reception to improve the Signal to Interference-plus-Noise Ratio (SINR) of the received signal. A Generalized Rake (G-Rake) receiver improves interference suppression performance over a conventional Rake receiver by increasing the sophistication of combining weight generation.
LIW receivers satisfy the requirements for type II receivers for the WCDMA downlink, as specified in the RAN4 of 3GPP. These requirements were formulated assuming a single transmit antenna at a base station. However, WCDMA defines two transmit diversity modes. Published U.S. patent application number 2005/0201447, METHOD AND APPARATUS FOR PARAMETER ESTIMATION IN A GENERALIZED RAKE RECEIVER, filed Mar. 12, 2004, assigned to the assignee of the present application and incorporated herein by reference in its entirety, discloses a solution for G-Rake receivers in a transmit diversity system. The solution describes a parametric approach to estimating an impairment covariance matrix used to form G-Rake combining weights. The parametric approach estimates the impairment covariance as a sum of terms, including a separate term for each transmit antenna.
This solution works well for an open-loop transmit diversity mode. In the open-loop mode, the impairment due to each transmit antenna during a particular symbol period is uncorrelated, since different symbols are transmitted from the different antennas. In a closed-loop mode, however, the mobile terminal specifies a phase offset, and the same symbol is transmitted by a primary antenna and simultaneously by a secondary antenna with the specified phase offset. In this case, the impairment due to each transmit antenna is highly correlated. This correlation may be exploited to improve interference suppression and hence, receiver performance.